Steam assisted gravity discharge (SAGD) refers to a widely used process where high pressure steam is injected into an injection well to melt bitumen or to generally reduce the viscosity of heavy oil to facilitate its removal. The bitumen or heavy oil and condensed steam flows by gravity to drain pipes buried below the oil deposit and the bitumen or oil is pumped out as an oil-water mixture. Once the oil-water mixture is pumped to the surface, a number of processes are utilized to treat the oil-water. First, oil is separated from the oil-water mixture to yield an oil product and produce water. The produced water is then treated to remove total dissolved solids and suspended solids. Various types of treatments can be employed such as filters for removing suspended solids and warm lime softeners or evaporators to remove dissolved solids. Cyclic Steam Simulation (CSS) process also works in the same principle as SAGD process with intermittent steam injection followed by oil-water mixture extraction.
There are several types of steam generators that can be utilized to generate steam for use in a SAGD process for example. One type of steam generator is referred to as the once through steam generator. Once through steam generators have a number of disadvantages or drawbacks. They tend to have high blowdown and hence this gives rise to thermal inefficiencies and water wastage. Once through steam generators typically utilize inline steam separators and this results in additional blow down and additional heat recovery equipment. Many once through steam generators are designed with refractory/insulated furnaces. These typically require substantial maintenance. In addition, once through steam generators have uncooled supports for supporting steam generation coils. This also leads to high maintenance. With once through steam generators the turn down is limited and they typically have very complex flow circuits to manage. Moreover, the steam capacity is limited to about 300,000 LB/HR. Typically once through steam generators require a relatively large footprint and the capital cost is high. When once through steam generators are used in heavy oil recovery processes such as commercial bitumen production, the resulting designs require numerous one through steam generation units and this results in high capital and operating costs.
A second type of steam generator is what is referred to as a drum boiler. Drum boilers have limited operating experience in heavy oil recovery processes and in particular, have not been widely used with feed water from an evaporator. Further, there is not a great deal of experience with drum boilers in handling upsets in water quality, a real concern for oil producers. Furthermore, with drum boilers it is expensive and time consuming to clean the tubes of the drum boiler. Finally, mechanical tube failures that result from water quality issues are expensive to repair.
Therefore, there is and continues to be a need for a steam generator design for use in heavy oil recovery processes that overcomes the shortcomings and disadvantages of once through steam generators and drum boilers.